In hydraulic circuits, differential pressure indicators are used to warn the operator that the filter is plugged and needs to be replaced or cleaned. These indicators sense the pressure differential between the fluid flow upstream from the filter and the fluid flow downstream from the filter. As the filter becomes increasingly plugged with contaminants filtered from the fluid, the restriction and the differential pressure between fluid upstream of the filter and fluid downstream of the filter increases.
Typically, these indicators incorporate a spring loaded piston. One side of the piston is plumbed to the dirty fluid on the upstream side of the filter and the other side of the piston is plumbed to the clean fluid on the downstream side of the filter. A spring opposes the movement of the piston until a particular predetermined differential pressure between the upstream and downstream sides of the filter exists. Once the differential pressure is sufficient to actuate the piston by overcoming the force of the spring, the piston can mechanically move or actuate a switch to either send an electrical signal by closing an electrical circuit or prevent an electrical signal from being sent by opening the electrical circuit to thereby alert the operator of the poor condition of the filter. Examples of this are shown in U.S. Pat. No. 3,704,458 to Buchanan et al.; U.S. Pat. No. 4,626,344 to Fick et al.; and U.S. Pat. No. 5,718,822 to Richter.
These differential pressure indicators work on a single set of logic such that they are designed for either opening an electrical circuit or closing an electrical circuit. Unfortunately, to manufacture either an indicator that opens a circuit or an indicator that closes a circuit requires significantly different components and/or design from the other. The present invention provides improvements in differential pressure indicators.